When excavating a cavity or drilling a wellbore in an underground formation from a surface location, a fluid mixture (e.g., a drilling fluid or mud) at an overbalanced hydrostatic pressure is typically used. An overbalanced pressure is a hydrostatic pressure in excess of the formation pore pressure along the entire length of the open wellbore wall. The overbalanced pressure drilling fluid helps to prevent wellbore wall caving, to consolidate loose formations, and to prevent the intrusion of an unwanted formation fluid, such as a "kick" of gas.
However, the overbalanced pressure drilling fluids also tend to intrude into permeable portions of the formation, such as productive intervals. This intrusion can damage the productive intervals, e.g., a water based drilling fluid causing swelling of a clay containing formation and the resulting loss of permeability. Damage to productive intervals by the drilling fluid may be shallow, e.g., a thin skin effect around the wellbore, or may extend radially deep into the formation.
When completing a well after drilling, e.g., gravel packing and perforating, an overbalanced pressure fluid or "kill fluid" is also typically used in the wellbore. Gravel packing is typically used in unconsolidated sand formations where sand would otherwise be produced along with the formation fluid during production. Perforation is typically used when the production flowrate would be otherwise be unacceptably low. The kill fluid typically used in these completion processes must similarly prevent uncontrolled well-flow and caving of the wellbore during the completion process.
If gravel packing is needed for sand control, the kill fluid also serves to entrain and carry the gravel into the face of the (sandy) formation. The flow of the entraining fluid may be under even greater hydrostatic pressure to move the gravel into the face being packed.
The even greater hydrostatic pressure of the entraining fluid tends to further intrude into and damage the productive intervals of the formation. Damage to the productive intervals may extend even further into the formation if a perforating process creates deep fractures and the entraining fluids are under high (overbalanced) pressure. The highly overbalanced pressure fluid may also fracture or otherwise damage the formation structure which, after removal of the overbalanced pressure, may collapse when the interval is produced at underbalanced pressure.
Although various kill fluid properties may be desirable to control during well completions, density is the most important. The kill fluid density must typically generate a hydrostatic pressure profile in the wellbore greater than the (hydrostatic) pore pressure profile in the formation. Aqueous based or other high density kill fluids and fluid mixtures are typically used.
Other entraining fluids may be more compatible and less damaging to a productive interval, but may not have adequate density to be used safely, e.g. light oil-based fluids for an oil bearing productive interval would require large quantities of light oil under very high surface pressures. Mixing these light oil fluids with other fluids may provide the desired density, but sacrifice viscosity, compatibility, or other desired properties of an entraining fluid.